Determining the rollover rate is very simple actually. All you need to do is find out the difference in the overnight interest rate between the 2 countries whose currency you are trading. Then multiply that percentage by the amount of the currency you are trading and divide it by 365. At this point you have the rollover in the base currency. If you want to convert it to dollars just multiply your total by the current rate of exchange to the dollar from the base currency.

So for example lets say you want to find out how much the rollover is on 100,000 GBP/JPY long. Start with the difference in the overnight rate 5.25% and multiply it by 100,000 then divide it by 365. It would look like this
100000 x .0525 = 5250 5350/365 = 14.38...

This is the amount of pounds per day if you wish to find the amount of dollars multiply the figure by 2.03 since that is the current exchange rate and you come up with $29.19 a day.

convert to dollars 14.38 x 2.03 = $29.19

If your not earning this much per day someone somewhere is earning money on your swap!